Asymmetric correlated electron switch operation

ABSTRACT

Subject matter disclosed herein may relate to correlated electron switches that are capable of asymmetric set or reset operations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/681,236, filed Aug. 18, 2017, titled “ASYMMETRIC CORRELATED ELECTRONSWITCH OPERATION,” which is a continuation of U.S. patent applicationSer. No. 14/850,213, filed Sep. 10, 2015, titled “ASYMMETRIC CORRELATEDELECTRON SWITCH OPERATION,” both of which being assigned to the Assigneeof claimed subject matter and incorporated herein by reference in theirentirety.

BACKGROUND Field

Subject matter disclosed herein may relate to a correlated electronswitch device.

Information

Integrated circuit devices, such as electronic switching devices, forexample, may be found in a wide range of electronic device types. Forexample, memory and/or logic devices may incorporate electronic switchesthat may be used in computers, digital cameras, cellular telephones,tablet devices, personal digital assistants, etc. Factors related toelectronic switching devices, such as may be incorporated in memoryand/or logic devices that may be of interest to a designer inconsidering suitability for any particular application may includephysical size, storage density, operating voltages, and/or powerconsumption, for example. Other example factors that may be of interestto designers may include cost of manufacture, ease of manufacture,scalability, and/or reliability. Also, there appears to be an everincreasing need for memory and/or logic devices that exhibitcharacteristics of lower power and/or higher speed.

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out and distinctlyclaimed in the concluding portion of the specification. However, both asto organization and/or method of operation, together with objects,features, and/or advantages thereof, it may best be understood byreference to the following detailed description if read with theaccompanying drawings in which:

FIG. 1a shows block diagram of an example embodiment of a correlatedelectron switch (CES) device comprising a correlated electron material,in accordance with an embodiment.

FIG. 1b depicts an example symbol for a correlated electron switch.

FIG. 2 is a schematic diagram of an equivalent circuit of a CES, inaccordance with an embodiment.

FIG. 3 shows a plot of current density versus voltage for a CES,according to an embodiment.

FIG. 4 is a plot illustrating symmetric operation of a CES deviceaccording to an embodiment.

FIGS. 5A through 5D are plots illustrating symmetric operation of a CESdevice according particular embodiments.

FIG. 6 is a plot illustrating asymmetric operation of a CES device inaccordance with an embodiment.

FIGS. 7A through 7I are diagrams illustrating structures of CES devicesaccording to particular embodiments.

Reference is made in the following detailed description to accompanyingdrawings, which form a part hereof, wherein like numerals may designatelike parts throughout to indicate corresponding and/or analogouscomponents. It will be appreciated that components illustrated in thefigures have not necessarily been drawn to scale, such as for simplicityand/or clarity of illustration. For example, dimensions of somecomponents may be exaggerated relative to other components. Further, itis to be understood that other embodiments may be utilized. Furthermore,structural and/or other changes may be made without departing fromclaimed subject matter. It should also be noted that directions and/orreferences, for example, such as up, down, top, bottom, and so on, maybe used to facilitate discussion of drawings and/or are not intended torestrict application of claimed subject matter. Therefore, the followingdetailed description is not to be taken to limit claimed subject matterand/or equivalents.

DETAILED DESCRIPTION

References throughout this specification to one implementation, animplementation, one embodiment, an embodiment and/or the like means thata particular feature, structure, and/or characteristic described inconnection with a particular implementation and/or embodiment isincluded in at least one implementation and/or embodiment of claimedsubject matter. Thus, appearances of such phrases, for example, invarious places throughout this specification are not necessarilyintended to refer to the same implementation or to any one particularimplementation described. Furthermore, it is to be understood thatparticular features, structures, and/or characteristics described arecapable of being combined in various ways in one or more implementationsand, therefore, are within intended claim scope, for example. Ingeneral, of course, these and other issues vary with context. Therefore,particular context of description and/or usage provides helpful guidanceregarding inferences to be drawn.

The terms, “and”, “or”, “and/or” and/or similar terms, as used herein,include a variety of meanings that also are expected to depend at leastin part upon the particular context in which such terms are used.Typically, “or” if used to associate a list, such as A, B or C, isintended to mean A, B, and C, here used in the inclusive sense, as wellas A, B or C, here used in the exclusive sense. In addition, the term“one or more” and/or similar terms is used to describe any feature,structure, and/or characteristic in the singular and/or is also used todescribe a plurality and/or some other combination of features,structures and/or characteristics. Likewise, the term “based on” and/orsimilar terms are understood as not necessarily intending to convey anexclusive set of factors, but to allow for existence of additionalfactors not necessarily expressly described. Of course, for all of theforegoing, particular context of description and/or usage provideshelpful guidance regarding inferences to be drawn. It should be notedthat the following description merely provides one or more illustrativeexamples and claimed subject matter is not limited to these one or moreillustrative examples; however, again, particular context of descriptionand/or usage provides helpful guidance regarding inferences to be drawn.

Particular aspects of the present disclosure incorporate correlatedelectron material (CEM) to form a correlated electron switch (CES), suchas, for example, in memory and/or logic devices. CES devices may also beutilized in other types of electronic circuits, such as, for example,filter circuits, as discussed more fully below. However, the scope ofclaimed subject matter is not limited in scope in these respects. Inthis context, a CES may exhibit a substantially abruptconductor/insulator transition arising from electron correlations ratherthan solid state structural phase changes (e.g., crystalline/amorphousin phase change memory (PCM) devices or filamentary formation andconduction in resistive RAM devices). In one aspect, a substantiallyabrupt conductor/insulator transition in a CES may be responsive to aquantum mechanical phenomenon, in contrast to melting/solidification orfilament formation, for example. Such a quantum mechanical transitionbetween conductive and insulative states, and/or between first andsecond impedance states, in a CES may be understood in any one ofseveral aspects. As used herein, the terms “conductive state”, “lowerimpedance state”, and/or “metal state” may be interchangeable, and/ormay at times be referred to as a “conductive/lower impedance state.”Similarly, the terms “insulative state” and “higher impedance state” maybe used interchangeably herein, and/or may at times be referred to as an“insulative/higher impedance state.”

In an aspect, a quantum mechanical transition of correlated electronswitch material between an insulative/higher impedance state and aconductive/lower impedance state may be understood in terms of a Motttransition. In a Mott transition, a material may switch from aninsulative/higher impedance state to a conductive/lower impedance stateif a Mott transition condition occurs. The Mott criteria is defined by(n_(C))1/³ a≈0.26, where n_(C) is a concentration of electrons and “a”is the Bohr radius. When a critical carrier concentration is achievedsuch that the Mott criteria is met, the Mott transition will occur andthe state of the CES will change from a higher resistance/highercapacitance state to a lower resistance/lower capacitance state.

In another aspect, the Mott transition is controlled by a localizationof electrons. When carriers are localized, the strong coulombinteraction between the electrons splits the bands of the CEM to createan insulator. When electrons are no longer localized, the weak coulombinteraction dominates and the band splitting is removed, resulting in ametal (conductive) band. This is sometimes explained as a “crowdedelevator” phenomenon. While an elevator has only a few people in it, thepeople can move around easily, which is analogous to a conductive/lowerimpedance state. While the elevator reaches a certain concentration ofpeople, on the other hand, the people can no longer move, which isanalogous to the insulative/higher impedance state. However, it shouldbe understood that this classical explanation provided for illustrativepurposes, like all classical explanations of quantum phenomenon, is onlyan incomplete analogy, and that claimed subject matter is not limited inthis respect.

In a further aspect, switching from an insulative/higher impedance stateto a conductive/lower impedance state may bring about a change incapacitance in addition to a change in resistance. That is, in anaspect, a CES may comprise the property of variable resistance togetherwith the property of variable capacitance. For example, in a metalstate, a CEM may have substantially zero electric field, and thereforesubstantially zero capacitance. Similarly, in an insulative/higherimpedance state (in which electron screening may be very imperfect dueto lower density of free electrons), an external electric field may becapable of penetrating the CEM and therefore the CEM will havecapacitance due to a physical change in the dielectric function of theCEM. Thus, for example, a transition from an insulative/higher impedancestate to a conductive/lower impedance state in a CES may result inchanges in both resistance and capacitance, in an aspect.

In an embodiment, a CES device may switch impedance states responsive toa Mott-transition in a majority of the volume of the CEM of a CESdevice. In an embodiment, a CES device may comprise a “bulk switch.” Asused herein, the term “bulk switch” refers to at least a majority volumeof a CEM of a CES device switching impedance states, such as responsiveto a Mott-transition. For example, in an embodiment, substantially allof a CEM of a CES device may switch from an insulative/higher impedancestate to a conductive/lower impedance state or from a conductive/lowerimpedance state to an insulative/higher impedance state responsive to aMott-transition. In an aspect, a CEM may comprise one or more transitionmetal oxides, one or more rare earth oxides, one or more oxides of oneor more f-block elements of the periodic table, one or more rare earthtransitional metal oxide perovskites, yttrium, and/or ytterbium,although claimed subject matter is not limited in scope in this respect.In an embodiment, a device, such as CES device, may comprise CEMincluding one or more materials selected from a group comprisingaluminum, cadmium, chromium, cobalt, copper, gold, iron, manganese,mercury, molybdenum, nickel, palladium, rhenium, ruthenium, silver, tin,titanium, vanadium, and zinc (which may be linked to a cation such asoxygen or other types of ligands), or combinations thereof, althoughclaimed subject matter is not limited in scope in this respect.

FIG. 1a shows an example embodiment 100 of a CES device comprising CEM,such as material 102, sandwiched between conductive terminals, such asconductive terminals 101 and 103. In an embodiment, a CES device, suchas CES device 100, may comprise a variable impeder device. As utilizedherein, the terms “correlated electron switch” and “variable impeder”may be interchangeable. At least in part through application of acritical voltage and a critical current between the terminals, such asbetween conductive terminals 101 and 103, the CEM, such as material 102,may transition between the aforementioned conductive/lower impedancestate and insulative/higher impedance state. As mentioned, CEM, such asmaterial 102, in a variable impeder device, such as variable impeder100, may transition between a first impedance state and a secondimpedance state due to a quantum mechanical transition of the correlatedelectron switch material as a result an applied critical voltage and anapplied critical current, as described in more detail below. Also, asmentioned above, a variable impeder device, such as variable impederdevice 100, may exhibit properties of both variable resistance andvariable capacitance.

In a particular embodiment, a variable impeder device, such as variableimpeder device 100, may comprise CEM that may transition between oramong a plurality of detectable impedance states based, at least inpart, on a transition of at least a majority portion of the CEM betweenan insulative/higher impedance state and a conductive/lower impedancestate due to a quantum mechanical transition of the correlated electronswitch material. For example, in an embodiment, a variable impederdevice may comprise a bulk switch, in that substantially all of a CEM ofa variable impeder device may switch from an insulative/higher impedancestate to a conductive/lower impedance state or from a conductive/lowerimpedance state to an insulative/higher impedance state responsive to aMott-transition. In this context, an “impedance state” means adetectable state of a variable impeder device that is indicative of avalue, symbol, parameter and/or condition, just to provide a fewexamples. In one particular embodiment, as described below, an impedancestate of a variable impeder device may be detected based, at least inpart, on a signal detected on terminals of the variable impeder devicein a read and/or sense operation. In another particular embodiment, asdescribed below, a variable impeder device may be placed in a particularimpedance state to represent or store a particular value, symbol, and/orparameter, and/or to achieve a particular capacitance value for thevariable impeder device by application of one or more signals acrossterminals of the variable impeder device in a “write” and/or “program”operation, for example. Of course, claimed subject matter is not limitedin scope to the particular example embodiments described herein.

FIG. 1b depicts an example symbol 110 that may be utilized, for example,in electrical circuit schematic diagrams to notate a CES and/or avariable impeder device. Example symbol 110 is meant to remind theviewer of the variable resistance and variable capacitance properties ofa CES and/or variable impeder device, such as variable impeder device100. Example symbol 110 is not meant to represent an actual circuitdiagram, but is merely meant as an electrical circuit diagram symbol. Ofcourse, claimed subject matter is not limited in scope in theserespects.

FIG. 2 depicts a schematic diagram of an equivalent circuit of anexample variable impeder device, such as variable impeder device 100. Asmentioned, a variable impeder device may comprise characteristics ofboth variable impedance and variable capacitance. For example, anequivalent circuit for a variable impeder device may, in an embodiment,comprise a variable resistor, such as variable resistor 210 in parallelwith a variable capacitor, such as variable capacitor 220. Of course,although a variable resistor 210 and variable capacitor 220 are depictedin FIG. 2 as comprising discrete components, a variable impeder device,such as variable impeder device 100, may comprise a substantiallyhomogenous CEM, such as CEM 102, wherein the CEM comprisescharacteristics of variable capacitance and variable resistance.

Table 1 below depicts an example truth table for an example variableimpeder device, such as variable impeder device 100.

TABLE 1 Correlated Electron Switch Truth Table Resistance CapacitanceR_(high)(V_(applied)) C_(high)(V_(applied)) R_(low)(V_(applied))C_(low)(V_(applied))~0

In an embodiment, example truth table 120 shows that a resistance of avariable impeder device, such as variable impeder device 100, maytransition between a lower resistance state and a higher resistancestate that is a function, at least in part, of a voltage applied acrossthe CEM. In an embodiment, a resistance of a lower resistance state maybe 10-100,000 times lower than a resistance of a higher impedance state,although claimed subject matter is not limited in scope in this respect.Similarly, example truth table 120 shows that a capacitance of avariable impeder device, such as variable impeder device 100, maytransition between a lower capacitance state, which for an exampleembodiment may comprise approximately zero, or very little, capacitance,and a higher capacitance state that is a function, at least in part, ofa voltage applied across the CEM. It should be noted that a variableimpeder is not a resistor, but rather comprises a device havingproperties of both variable capacitance and variable resistance. In anembodiment, resistance and/or capacitance values depend, at least inpart, on an applied voltage.

FIG. 3 shows a plot of current density versus voltage acrosselectrically conductive terminals, such as electrically conductiveterminals 101 and 103, for a variable impeder device, such as examplevariable impeder device 100, according to an embodiment. Based, at leastin part, on a voltage applied to terminals of a variable impeder device(e.g., in a write operation), such as variable impeder device 100, aCEM, such as CEM 102, may be placed in a conductive/lower impedancestate or an insulative/higher impedance state. For example, applicationof a voltage V_(reset) and current density J_(reset) may place thevariable impeder device in an insulative/higher impedance state, andapplication of a voltage V_(set) and a current density J_(set) may placethe variable impeder device in a conductive/lower impedance state. Thatis, a “set” operation may place a variable impeder device, such asvariable impeder device 100, into a conductive/lower impedance state,and a “reset” operation may place a variable impeder device, such asvariable impeder device 100, into an insulative/higher impedance state,in an embodiment. Following placement of the variable impeder device ina lower impedance state or a higher impedance state, the particularstate of the variable impeder device may be detected at least in part byapplication of a voltage V_(read) (e.g., in a read operation) anddetection of a current or current density at terminals, such aselectrically conductive terminals 101 and 103, of a variable impederdevice, such as variable impeder device 100.

In an embodiment, a CEM of a variable impeder device may include, forexample, any TMO, such as, for example, peroskovites, Mott insulators,charge exchange insulators, and/or Anderson disorder insulators. In aparticular embodiment, a CES device may be formed from materials such asnickel oxide, cobalt oxide, iron oxide, yttrium oxide and peroskovitessuch as Cr doped strontium titanate, lanthanum titanate, and themanganite family including praesydium calcium manganite, and praesydiumlanthanum manganite, to provide a few examples. In an embodiment, oxidesincorporating elements with incomplete d and f orbital shells mayexhibit sufficient impedance switching properties for use in a CESdevice. In an embodiment, a CES may be prepared without electroforming.Other embodiments may employ other transition metal compounds withoutdeviating from claimed subject matter. For example, {M(chxn)₂Br}Br₂where M may comprise Pt, Pd, or Ni, and chxn comprises1R,2R-cyclohexanediamine, and other such metal complexes may be usedwithout deviating from the scope of claimed subject matter.

In one aspect, the variable impeder device of FIG. 1 may comprisematerials that comprise TMO metal oxide variable impedance materials,though it should be understood that these are exemplary only, and arenot intended to limit the scope of claimed subject matter. Particularimplementations may employ other variable impedance materials as well.Nickel oxide, NiO, is disclosed as one particular TMO. NiO materialsdiscussed herein may be doped with extrinsic ligands, which maystabilize variable impedance properties by passivating the interfacingand allowing for adjustable voltages and impedances, in an embodiment.In a particular embodiment, NiO variable impedance materials disclosedherein may include a carbon containing ligand, which may be indicated byNiO(C_(x)). Here, one skilled in the art may determine a value of x forany specific carbon containing ligand and any specific combination ofcarbon containing ligand with NiO simply by balancing valences, in anembodiment. In another particular example embodiment, NiO doped withextrinsic ligands may be expressed as NiO(L_(x)), where L_(x) is aligand element or compound and x indicates a number of units of theligand for one unit of NiO. One skilled in the art may determine a valueof x for any specific ligand and any specific combination of ligand withNiO or any other transition metal simply by balancing valences, in anembodiment.

According to an embodiment, if sufficient bias is applied (e.g.,exceeding a band-splitting potential) and the aforementioned Mottcondition is met (injected electron holes=the electrons in the switchingregion), the variable impeder device may rapidly switch from aconductive/lower impedance state to an insulator state via the Motttransition. This may occur at point 308 of the plot in FIG. 3. At thispoint, electrons are no longer screened and become localized. Thiscorrelation splits the bands to form an insulator. While the CEM of thevariable impeder device is still in the insulative/higher impedancestate, current may generated by transportation of holes. If enough biasis applied across terminals of the variable impeder device, electronsmay be injected into a metal-insulator-metal (MIM) diode over thepotential barrier of the MIM device. If enough electrons have beeninjected and enough potential is applied across terminals to achieve aset condition, an increase in electrons may screen electrons and removea localization of electrons, which may collapse the band-splittingpotential forming a metal, thereby placing the variable impeder devicein a conductive/lower impedance state.

According to an embodiment, current in a CEM of a variable impederdevice may be controlled by an externally applied “compliance” conditiondetermined based, at least in part, on the external current limitedduring a write operation to achieve a set condition to place thevariable impeder device in a conductive/lower impedance state. Thisexternally applied compliance current also sets the subsequent resetcondition current density requirement. As shown in the particularimplementation of FIG. 3, a current density J_(comp) applied during awrite operation at point 316 to place the variable impeder device in aconductive/lower impedance state may determine a compliance conditionfor placing the variable impeder device in an insulative/higherimpedance state in a subsequent write operation. As shown, the CEM ofthe variable impeder device may be subsequently placed in aninsulative/higher impedance state by application of a current densityJ_(reset)≥J_(comp) at a voltage V_(reset) at point 308, wherein J_(comp)may be externally applied, in an embodiment.

A compliance current, such as an externally applied compliance current,therefore may set a number of electrons in a CEM of a variable impederdevice which are to be “captured” by holes for the Mott transition. Inother words, a current applied in a write operation to place a variableimpeder device in a conductive/lower impedance state may determine anumber of holes to be injected to the CEM of the variable impeder devicefor subsequently transitioning the variable impeder device to aninsulative/higher impedance state. As discussed more fully below, acompliance current may be applied dynamically.

As pointed out above, a transition to an insulative/higher impedancestate may occur in response to a Mott transition at point 308. Aspointed out above, such a Mott transition may occur at a condition in aCEM of a variable impeder device in which a concentration of electrons nequals a concentration of electron holes p. This condition occurs whenthe following Mott criteria is met, as represented by expression (1) asfollows:

$\begin{matrix}{{{\lambda_{TF}n^{\frac{1}{3}}} = { C \sim 0.26}}{n = ( \frac{C}{\lambda_{TF}} )^{3}}} & (1)\end{matrix}$where:

λ_(TF) is a Thomas Fermi screening length; and

C is a constant which equals approximately 0.26 for the Mott transition.

According to an embodiment, a current or current density in a region 304of the plot shown in FIG. 3 may exist in response to an injection ofholes from a voltage signal applied across terminals, such as terminals101 and 103, of a variable impeder device, such as variable impederdevice 100. Here, injection of holes may meet a Mott transitioncriterion for the conductive to insulator transition at current IAN as acritical voltage VAN is applied across terminals, such as terminal 101and 103, of a variable impeder device, such as variable impeder device100. This may be modeled according to expression (2) as follows:

$\begin{matrix}{{{I_{MI}( V_{MI} )} = {\frac{{dQ}( V_{MI} )}{dt} \approx \frac{Q( V_{MI} )}{t}}}{{Q( V_{MI} )} = {{qn}( V_{MI} )}}} & (2)\end{matrix}$Where Q(V_(MI)) is the charge injected (hole or electron) and is afunction of the applied voltage. As used herein, the notation “MI”signifies a metal-to-insulator transition, and the notation “IM”signifies an insulator-metal transition. That is, “V_(MI)” refers to acritical voltage and “I_(MI)” refers to a critical current to transitiona CEM from a conductive/lower impedance state to an insulative/higherimpedance state. Similarly, “V_(IM)” refers to a critical voltage and“I_(IM)” refers to a critical current to transition a CEM from aninsulative/higher impedance state to a conductive/lower impedance state.

Injection of holes to enable a Mott transition may occur between bandsand in response to critical voltage V_(MI). and critical current IAN. Byequating electron concentration n with the needed charge concentrationto result in a Mott transition by holes injected by IAN in expression(2) according to expression (1), a dependency of such a critical voltageV_(MI) on Thomas Fermi screening length λ_(TF) may be modeled accordingto expression (3) as follows:

$\begin{matrix}{{{I_{MI}( V_{MI} )} = {\frac{Q( V_{MI} )}{t} = {\frac{{qn}( V_{MI} )}{t} = {\frac{q}{t}( \frac{C}{\lambda_{TF}} )^{3}}}}}{{J_{reset}( V_{MI} )} = {{J_{MI}( V_{MI} )} = {\frac{I_{MI}( V_{MI} )}{A_{CEM}} = {\frac{q}{A_{CEM}t}( \frac{C}{\lambda_{TF}( V_{MI} )} )^{3}}}}}} & (3)\end{matrix}$Wherein A_(CEM) is a cross-sectional area of a CEM, such as CEM 102, ofa variable impeder device, such as variable impeder device 100, andwherein J_(reset)(V_(MI)), depicted at point 308 of example plot 300, isa current density through the CEM, such as CEM 102, to be applied to theCEM at a critical voltage V_(MI) to place the CEM of the variableimpeder device in an insulative/higher impedance state. In anembodiment, a CEM may be switched between a conductive/lower impedancestate and an insulative/higher impedance state at least in part by adisproportionation reaction.

According to an embodiment, a CEM, such as CEM 102, of a variableimpeder device, such as variable impeder device 100, may be placed in aconductive/lower impedance state (e.g., by transitioning from aninsulative/higher impedance state) by injection of a sufficient numberof electrons to satisfy a Mott transition criteria.

In transitioning a CEM of a variable impeder device to aconductive/lower impedance state, as enough electrons have been injectedand the potential across terminals of the variable impeder deviceovercomes a critical switching potential (e.g., V_(set)), injectedelectrons begin to screen and unlocalize double-occupied electrons toreverse a disproportion reaction and closing the bandgap. A currentdensity J_(set)(V_(MI)), depicted at point 314 of FIG. 3, fortransitioning the CEM of the variable impeder device to theconductive/lower impedance state in a metal-insulator Mott transition ata critical voltage V_(MI) enabling transition to the conductive/lowerimpedance state may be represented according to expressions (4) asfollows:

$\begin{matrix}{{{I_{MI}( V_{MI} )} = {\frac{{dQ}( V_{MI} )}{dt} \approx \frac{Q( V_{MI} )}{t}}}{{Q( V_{MI} )} = {{qn}( V_{MI} )}}{{I_{MI}( V_{MI} )} = {\frac{Q( V_{MI} )}{t} = {\frac{{qn}( V_{MI} )}{t} = {\frac{q}{t}( \frac{C}{a_{B}\;} )^{3}}}}}{{J_{set}( V_{IM} )} = {{J_{injection}( V_{IM} )} = {{J_{IM}( V_{IM} )} = {\frac{I_{IM}( V_{IM} )}{A_{CEM}} = {\frac{q}{A_{CEM}t}( \frac{C}{a_{B}} )^{3}}}}}}} & (4)\end{matrix}$where:

a_(B) is a Bohr radius.

According to an embodiment, a “read window” 302 for detecting a memorystate of a variable impeder device in a read operation may be set out asa difference between a portion 306 the plot of FIG. 3 while the CEM ofthe variable impeder device is in an insulative/higher impedance stateand a portion 304 of the plot FIG. 3 while the CEM of the variableimpeder device is in a conductive/lower impedance state at a readvoltage V_(read). In a particular implementation, read window 302 may beused to determine the Thomas Fermi screening length λ_(TF) of a CEM,such as correlated electron switch material 102, of a variable impederdevice, such as variable impeder device 100. For example, at a voltageV_(reset), current densities J_(reset) and J_(set) may be related toaccording to expression (5) as follows:

$\begin{matrix}{{\lambda_{TF}( {@V_{reset}} )} = {a_{B}( \frac{J_{reset}}{J_{off}} )}^{\frac{1}{3}}} & (5)\end{matrix}$wherein J_(off) represents a current density of a CEM in aninsulative/higher impedance state at V_(reset). See, for example, point309 of FIG. 3.

In another embodiment, a “write window” 310 for placing a CEM ofvariable impeder device in an insulative/higher impedance orconductive/lower impedance state in a write operation may be set out asa difference between V_(reset) and V_(set). Establishing|V_(set)|>|V_(reset)| may enable a switch between the conductive/lowerimpedance and insulative/higher impedance state. V_(reset) may compriseapproximately the band splitting potential caused by the correlation andV_(set) may comprise approximately twice the band splitting potential,such that the read window may comprise approximately the band-splittingpotential. In particular implementations, a size of write window 310 maybe determined, at least in part, by materials and doping of the CEM ofthe variable impeder device.

In an embodiment, a process for reading a value represented as animpedance state of a variable impeder device, such as variable impederdevice 100, may comprise a voltage being applied to a CEM of a variableimpeder device. At least one of a current and/or current density withina CEM of a variable impeder device may be measured, and an impedancestate of a CEM of a variable impeder device may be determined, at leastin part, on the measured current and/or current density, in anembodiment.

Additionally, in an embodiment, an impedance of an impedance state maydepend at least in part on a combination of a capacitance and aresistance of a CEM of a variable impeder device. In an embodiment, thedetermined impedance state may comprise one of a plurality of impedancestates. A first impedance state may comprise a lower resistance andlower capacitance, and a second impedance state may comprise a higherresistance and a higher capacitance, for example. Also, in anembodiment, a ratio of the impedances of the plurality of impedancestates may be proportional to a physical property of the CEM of thevariable impeder device. In an embodiment, the physical property of theCEM of the variable impeder device may comprise at least one of a ThomasFermi screening length and a Bohr radius. Further, in an embodiment,individual impedance states of the plurality of impedance states may beassociated with a data value. Additionally, in an embodiment, adifference in current between a first impedance state and a secondimpedance state at a predetermined voltage provides an indication of aread window. However, claimed subject matter is not limited in scope inthese respects.

In an embodiment, a plurality of electrons may be provided to a CEM of avariable impeder device such that the CEM enters a first impedancestate. A plurality of holes may be provided to the CEM such that the CEMenters a second impedance state. Also, in an embodiment, the pluralityof electrons may cause a voltage across the CEM to be greater than a setvoltage threshold, and the plurality of holes may cause the voltageacross the CEM to be equal to or greater than a reset voltage threshold.Further, in an embodiment, a voltage across the CEM may cause a currentdensity in the CEM to be equal to or greater than a set current densityand/or a set current, and a voltage across the CEM may cause a currentdensity in the CEM to be equal to or greater than a reset currentdensity and/or a reset current.

Also, in an embodiment, a set voltage across the CEM and a set currentdensity through a CEM of a variable impeder device may be exceeded.Additionally, a reset voltage across a CEM and a reset current densitythrough a CEM of a variable impeder device may be exceeded. Further, inan embodiment, individual impedance states of a plurality of impedancestates may be associated with a data value.

In an embodiment, at least one of a reset voltage, a set voltage, and adifference between the set voltage and the reset voltage areproportional to a physical property of a CEM of a variable impederdevice. A physical property of a CEM may include at least one of astrong electron potential due to localization, and/or a correlation ofelectrons, for example. Also, in an embodiment, a difference in the setvoltage and the reset voltage may provide an indication of a size of atleast one of a write/program window.

According to an embodiment, a write operation as discussed above may becharacterized by a particular voltage condition and a particular currentcondition sufficient to place a CES in a particular impedance state. Forexample, a set operation to place a CES in a low impedance or conductivestate may be characterized by a “set voltage condition” (e.g., V_(set))and a “set current condition” (I_(set)). For example, point 314 in theplot of FIG. 3 indicates a particular set voltage condition along avoltage axis and a particular set current condition along a current orcurrent density axis. Similarly, a reset operation to place a CES in ahigh impedance or insulative state may be characterized by a “resetvoltage condition” (e.g., V_(reset)) and a “reset current condition”(I_(reset)). Additionally, as pointed out above, a particular resetcurrent condition for a CES may be determined based, at least in part,on a compliance current (I_(comp)) applied at point 316 in a previousset operation.

As may be observed from the plot of FIG. 3, a programming signal toplace a CES in a low impedance or conductive state may satisfy a setvoltage condition having a positive polarity and a set current conditionhaving a positive polarity. Likewise a programming signal to place a CESin a high impedance or insulative state may satisfy a reset voltagecondition having a positive polarity and a reset current conditionhaving a positive polarity. In particular embodiments, as illustrated inFIG. 4, write operation may be performed by meeting alternative voltageand current conditions in different polarities. In this context,operation in the voltage versus current plot of FIG. 4 with current andvoltage both having a positive polarity places operation in the firstquadrant while operation with current and voltage having a negativepolarity places operation in the third quadrant. For example, point 408may define a first set voltage condition of V_(set_1) and a first setcurrent condition I_(set_1) in a first quadrant while point 410 maydefine a second set voltage condition of V_(set_2) and a second resetcurrent condition I_(set_2) in a third quadrant. In other words aparticular CES device having operational characterized by the plot ofFIG. 4 may be placed in a low impedance or conductive state byapplication of a programming signal having a voltage V_(set_1) and acurrent I_(set_1) (at point 408), or by application of a programmingsignal having a voltage V_(set_2) and a current I_(set_2) (at point410).

Likewise, point 402 may define a first reset voltage condition ofV_(reset_1) and a first reset current condition I_(reset_1) in a firstquadrant while point 404 may define a second reset voltage condition ofV_(reset_2) and a second reset current condition I_(reset_2) in a thirdquadrant. In other words a particular CES device having operationalcharacterized by the plot of FIG. 4 may be placed in a high impedance orinsulative state by application of a programming signal having a voltageV_(reset_1) and a current I_(reset_1) (at point 402), or by applicationof a programming signal having a voltage V_(reset_2) and a currentI_(reset_2) (at point 404). Here, first and second reset currentconditions I_(reset_1) and I_(reset_2) may be determined based on amagnitude of a compliance current applied in a previous set operation ateither point 406 or 412.

In the particular embodiment of FIG. 4, a CES device may becharacterized as having symmetric voltage and current conditions forwrite operations to place the CES in a particular impedance state. Here,as discussed above, first set voltage and current conditions (V_(set_1)and I_(set_1)) are symmetric with second set voltage and currentconditions (V_(set_2) and I_(set_2)) in that |V_(set_1)|≈|V_(set_2)| and∥_(set_1)|≈|I_(set_2)|. Likewise, first reset voltage and currentconditions (V_(reset_1) and I_(reset_1)) are symmetric with second setvoltage and current conditions (V_(reset_2) and I_(reset_2)) in that|V_(reset_1)|≈|V_(reset_2)| and |I_(reset_1)|≈|I_(reset_2)|. In thiscontext, symmetric voltage conditions for a particular write operationare polar opposite voltages of substantially the same magnitude.Similarly, symmetric current conditions for a particular write operationare polar opposite currents of substantially the same magnitude.

In particular embodiments, FIGS. 5A through 5D are plots illustratingsymmetric operation of a CES device according particular embodimentssuch as the CES device discussed above with reference to FIG. 4. Here,it may observed that a set operation and a reset operation may occur inthe same or different quadrants of a voltage vs. current mapping. InFIG. 5A, for example, set voltage and current conditions at point 504(V_(set_1) and I_(set_1)) are in the first quadrant as reset voltage andcurrent conditions at point 502 (V_(reset_1) and I_(reset_1)). Similarlyin FIG. 5B, set voltage and current conditions at point 510 (V_(set_2)and I_(set_2)) are in the third quadrant as reset voltage and currentconditions at point 508 (V_(reset_2) and I_(reset_2)) are also in thethird quadrant. In contrast, in FIG. 5C, set voltage and currentconditions at point 516 (V_(set_2) and I_(set_2)) are in the thirdquadrant while reset voltage and current conditions at point 514(V_(reset_1) and I_(reset_1)) are in the first quadrant. Also, in FIG.5D, set voltage and current conditions at point 522 (V_(set_1) andI_(set_1)) are in the first quadrant while reset voltage and currentconditions at point 520 (V_(reset_2) and I_(reset_2)) are in the thirdquadrant. As may be observed, set voltage and current conditions in thefirst quadrant in FIGS. 5A and 5D at points 504 and 522 are symmetricwith set voltage and current conditions in the third quadrant in FIGS.5B and 5C. Similarly, reset voltage and current conditions in the firstquadrant in FIGS. 5A and 5C at points 502 and 524 are symmetric withreset voltage and current conditions in the third quadrant in FIGS. 5Band 5D at points 508 and 520. It should be understood that a set orreset operations may occur either the first or third quadrantsregardless of whether a most recent write operation occurred in thefirst quadrant of the third quadrant.

FIG. 6 shows plots illustrating asymmetric operation of a CES device inaccordance with an embodiment. In a first quadrant of a voltage vscurrent plot for write operations according to an embodiment, first setvoltage and current conditions are shown at point 610 (V_(set_1) andI_(set_1)) and first reset voltage and current conditions are shown atpoint 606 (V_(reset_1) and I_(reset_1)). The third quadrant of the plotof FIG. 6 shows three alternative plots with corresponding alternativesecond set voltage and current conditions, and corresponding secondreset voltage and current conditions. A first alternative plot definessecond set voltage and current conditions at point 622 (V_(set_2) andI_(set_2)), and second reset voltage and current conditions at point 616(V_(reset_2) and I_(reset_2)). For this particular alternative plot, itmay be observed that the first set voltage and current conditions ofpoint 622 are symmetric with the second set voltage and currentconditions of point 610 in the first quadrant (e.g.,|V_(set_1)|≈|V_(set_2)| and |I_(set_1)|≈|I_(set_2)|). Likewise, it maybe observed that the first reset voltage and current conditions of point616 are symmetric with the second set voltage and current conditions ofpoint 606 in the first quadrant (e.g., |V_(reset_1)|≈|V_(reset_2)| and|I_(reset_1)|≈|I_(reset_2)|).

A second alternative plot in the third quadrant defines second setvoltage and current conditions at point 614, and second reset voltageand current conditions at point 612. Here, it may be observed that thesecond set voltage and current conditions at point 614 in the thirdquadrant are asymmetric with the first set voltage and currentconditions at point 610 in the first quadrant in that|V_(set_1)|>>|V_(set_2)| and |I_(set_1)|>>|I_(set_2)|. Likewise, it maybe observed that the second reset voltage and current conditions atpoint 612 in the third quadrant are asymmetric with the first resetvoltage and current conditions at point 606 in the first quadrant inthat |V_(reset_1)|>>|V_(reset_2)| and |I_(reset_1)|>>|I_(reset_2)|.

A third alternative plot in the third quadrant defines second setvoltage and current conditions at point 620, and second reset voltageand current conditions at point 618. Here, it may be observed that thesecond set voltage and current conditions at point 620 in the thirdquadrant are asymmetric with the first set voltage and currentconditions at point 610 in the first quadrant in that|V_(set_1)|<<|V_(set_2)| and |I_(set_1)|<<|I_(set_2)|. Likewise, it maybe observed that the second reset voltage and current conditions atpoint 618 in the third quadrant are asymmetric with the first resetvoltage and current conditions at point 606 in the first quadrant inthat |V_(reset_1)|<<|V_(reset_2)| and |I_(reset_1)|<<|I_(reset_2)|.

According to an embodiment, and with reference to nomenclature for setand reset voltage and current conditions established in FIG. 4, firstand second set voltage conditions for a CES device are “asymmetric” if adifference between |V_(set_1)| and |V_(set_2)| is large enough tosignificantly or measurably affect operation or behavior of a circuitimplementing the CES device. Similarly, first and second set currentconditions are similarly asymmetric if a difference between |I_(set_1)|and |I_(set_2)| is large enough to significantly or measurably affectoperation or behavior of a circuit implementing the CES device. Firstand second reset voltage conditions, and first and second reset currentconditions of a CES device are similarly asymmetric if a differencebetween |V_(reset_1)| and |V_(reset_2)|, or a difference between|I_(reset_1)| and |I_(reset_2)| is large enough to significantly ormeasurably affect operation or behavior of a circuit incorporating theCES device.

In a particular implementation, a CES may be formed by coupling a CEMsubstrate between conductive terminals. According to an embodiment, aprocess for manufacturing a CES device may affect a composition ofstructure of a CEM substrate so as to provide a CES having asymmetricvoltage or current conditions for a set or reset operation as describedabove. FIGS. 7A through 7I are diagrams illustrating structures of CESdevices according to particular embodiments. FIG. 7A shows a structureof a CES comprising CEM substrate (e.g., a transition metal oxide)formed between terminals 702 and 710. A center portion 706 of the CEM ismaintained in an intrinsic state while equal portions 704 and 708 aredoped as P-type regions. Here, the symmetry of the structure of the CEMbetween terminals 702 and 710 may provide symmetric set voltage andcurrent conditions, and symmetric reset voltage and current conditionsas discussed above (e.g., as illustrated in the plot of FIG. 4). Asdescribed below in particular implementations, altering the compositionor structure of CEM between terminals may introduce asymmetric behaviorin set and reset operations.

Asymmetries described above may be introduced in a CES device (e.g., ina manufacturing process) using any one of several techniques to affect astructure or composition of a CEM substrate of CES. For example, as abias and injected charge is applied to terminals of a CES device toplace the CES device in a particular impedance state, physicalproperties or structure of the CES device may affect a rate of carrierinjection. In a set operation to place a CES device in a low impedanceor conductive state, for example, the CES device may be controlled viaelectron injection over a metal/insulator barrier.

In one implementation, the structure or composition of CES device ofFIG. 7A may be altered as shown in FIG. 7B to introduce an asymmetry inset or reset voltage or current conditions. As shown in FIG. 7B, region714 is more heavily P-type doped than region 718. In anotherimplementation as shown in FIG. 7C, a region 724 is P-typed doped,region 728 is N-type doped and region 726 is maintained in an intrinsicstate to provide a PIN structure. In such a PIN structure, injection ofelectrons may be amplified by injection via N-type doped region 728.This may tune switching behavior in a CES device by introducingdifferent set voltage and current conditions and/or different resetvoltage and current conditions in a reverse polarity (e.g., thirdquadrant).

FIGS. 7D and 7E illustrate an example of how asymmetric operation may beintroduced in an IPI device. FIG. 7D shows a CES device comprisingequally formed intrinsic regions 734 and 738 separated by a P-type dopedregion 736. As shown in FIG. 7E, asymmetric set or reset voltage orcurrent conditions may be introduced by replacing intrinsic region 738with a P or P+ doped region 748. Additionally, asymmetric operation maybe introduced by using asymmetric dimensions and/or asymmetric doping asillustrated in FIGS. 7F through 7I. In the CES device shown FIG. 7F, forexample, an asymmetric size of P-type doped portions 754 and 758separated by intrinsic region 756 may introduce asymmetries. This may befurther altered by introducing gradient in a concentration of P-typedoping as illustrated in FIG. 7G. Here, a concentration of P-type dopingin region 768 is highest toward terminal 770 and lowest toward intrinsicregion 766 according to a gradient. In FIG. 7H, P-type doped regions 782and 786 may be the same dimensions. However, asymmetry of set or resetvoltage or current conditions may be introduced by introduced byapplying a gradient to a concentration in P-type doping in regions 782and 786. As shown, a higher P-type doping concentration in region 782occurs toward intrinsic region 784 and a lower P-type dopingconcentration occurs toward terminal 780, and a higher P-typeconcentration in region 786 occurs toward terminal 788 and a lowerP-type doping concentration occurs toward intrinsic region 784. FIG. 7Ishows a single P-type doped region 794 between terminals 790 and 792. AP-type doping concentration gradient provides a highest concentrationtoward terminal 792 and a lowest P-type doping concentration towardterminal 790.

As described previously, in an embodiment, a voltage may be applied to aCEM of a CES device. Also, in an embodiment, at least one of a currentdensity and/or a current within the CEM may be measured, and animpedance state of the CES dependent on the measured current and/orcurrent density may be determined. In an embodiment, the impedance statemay be dependent on a combination of a capacitance and a resistance ofthe CEM. Further, in an embodiment, the impedance state may comprise oneof a plurality of impedance states, wherein a first of the plurality ofimpedance states has a lower impedance than a second of the plurality ofimpedance states. In an embodiment, the first impedance state may have alower resistance and a lower capacitance, and the second impedance statemay have a higher resistance and a higher capacitance. Additionally, inan embodiment, a ratio of impedances of the plurality of impedancestates may be proportional to a physical property of the CEM. Thephysical property of the CEM may include, for example, a Thomas Fermiscreening length and/or a Bohr radius. Also, in an embodiment,individual impedance states of the plurality of impedance states may beassociated with a data value. Further, a difference in current betweenthe first impedance state and the second impedance state at a determinedvoltage may provide an indication of a read window, in an embodiment.

As also described previously, in an embodiment, a plurality of electronsmay be provided to a CEM of a CES device such that the CES enters afirst impedance state, and a plurality of holes may be provided to theCEM such that the CES enters a second impedance state. Also, in anembodiment, the plurality of electrons may cause a voltage across theCEM to be greater than a set voltage condition, and the plurality ofholes may cause the voltage across the CEM to be equal to or greaterthan a reset voltage condition. Additionally, the voltage across the CEMmay cause a current density in the CEM to be equal to or greater than aset current density and/or a set current condition, and the voltageacross the CEM may cause the current density in the CEM to be equal toor greater than a reset current density and/or a reset current, in anembodiment. Further, in an embodiment, a set voltage across the CEM anda set current density through the CEM may be exceeded, and a resetvoltage across the CEM and a reset current density through the CEM maybe exceeded, in an embodiment. Also, in an embodiment, individualimpedance states may be associated with a data value. Additionally, atleast one of the reset voltage, the set voltage, and a differencebetween the set voltage and the reset voltage may be proportional to aphysical property of the CEM, wherein the physical property of the CEMmay include at least one of a strong electron potential due tolocalization and/or a correlation of the electrons, in an embodiment.Further, in an embodiment, the difference between the set voltage andthe reset voltage may provide an indication of a size of at least one ofa write window and/or a programming window.

In a further embodiment, as described previously, a plurality ofelectrons may be provided to a CEM of a CES device such that a currentand/or current density within the CEM exceeds a first threshold and avoltage across the CEM exceeds a second threshold. Further, in anembodiment, switching from a first impedance state to a second impedancestate may result from the current and/or current density exceeding thesecond threshold. The first impedance state may have a higher resistanceand a higher capacitance, and the second impedance state has a lowerresistance and a lower capacitance, in an embodiment. Additionally, thefirst threshold may be dependent on at least one of a current and/orcurrent density required to enable a Mott transition in the CEM, avoltage required to inject electrons over a metal insulator barrierwithin the CEM, and/or a voltage greater or equal to twice theband-splitting potential, in an embodiment.

An another embodiment, a plurality of electrons may be provided to a CEMof a CES device such that a concentration of electrons within the CEMexceeds a threshold, and a switching from a first impedance state to asecond impedance state may occur as a result of the concentration ofelectrons exceeding the threshold for a Mott transition. Further, in anembodiment, at least one of the plurality of electrons may be recombinedwith at least one of a plurality of holes within the CEM to enable theswitching from the first impedance state to the second impedance state.In an embodiment, the concentration of the plurality of electrons and/orholes may be dependent at least in part on at least one physicalproperty associated with the CEM. The at least one physical property mayinclude, for example the Bohr radius. Further, in an embodiment, thethreshold may be dependent at least in part on a current and/or currentdensity required to enable the Mott transition. Also, in an embodiment,a resistance and/or capacitance (or impedance) of the CEM may besubstantially different, such as a result of the Mott transition, forexample. Additionally, a switching from the first impedance state to thesecond impedance state may be caused by a disproportionation reaction,in an embodiment.

As discussed above, a CEM of a CES device may be provided with aplurality of holes such that a concentration of holes within the CEMexceeds a threshold, and switching from a first impedance state to asecond impedance state may occur as a result of the concentration ofholes exceeding the threshold, in an embodiment. The first impedancestate may comprise a lower resistance, lower capacitance state, and thesecond impedance state may comprise a higher resistance, highercapacitance state, for example. Also, in an embodiment, the thresholdmay depend, at least in part, on at least one of a current and/orcurrent density required to enable a Mott-like transition within theCEM, and/or a voltage greater or equal to the band-splitting potential.Additionally, in an embodiment, the threshold may be dependent on acurrent and/or currently density required to enable a Mott-liketransition. In an embodiment, at least one of the plurality of holes maybe recombined with a respective at least one of a plurality of electronswithin the CEM to enable switching from the first impedance state to thesecond impedance state. Also, a concentration of the plurality ofelectrons and/or holes may be dependent at least in part on at least onephysical property associated with the CEM. The at least one physicalproperty may include, for example, a Thomas Fermi screening length.Further, in an embodiment, the switching from the first impedance stateto the second impedance state may be caused by a disproportionationreaction. Also, in an embodiment, resistance and/or capacitance of theCEM may be substantially different, such as between the first impedancestate and the second impedance state, for example.

As also discussed previously, a variable impeder device may comprise aCEM capable of operating in a first impedance state and a secondimpedance state. In an embodiment, the first impedance state maycomprise a lower resistance, lower capacitance state, and the secondimpedance state may comprise a higher resistance, higher capacitancestate. Also, in an embodiment, a change in the capacitance may bedependent on at least one material property associated with the CEM. Inan embodiment, the CEM may comprises one or more of: one or moretransition metal oxides, one or more rare earth oxides, one or moreoxides of one or more f-block elements of the periodic table, one ormore rare earth transitional metal oxide perovskites, yttrium, and/orytterbium. Additionally, in an embodiment, a transition from the firstimpedance state to the second impedance state of the variable impederdevice may depend at least in part on an applied critical bias and acritical current/current density.

In the preceding description, various aspects of claimed subject matterhave been described. For purposes of explanation, specifics, such asamounts, systems and/or configurations, as examples, were set forth. Inother instances, well-known features were omitted and/or simplified soas not to obscure claimed subject matter. While certain features havebeen illustrated and/or described herein, many modifications,substitutions, changes and/or equivalents will now occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all modifications and/or changes as fallwithin claimed subject matter.

What is claimed is:
 1. A method comprising: applying a first programmingsignal to first and second terminals of a correlated electron switch(CES) device to meet a first voltage condition or a first currentcondition, or a combination thereof, for placement of the CES device ina first impedance state, wherein the CES device comprises one or morelayers of a correlated electron material (CEM) disposed between thefirst and second terminals; applying a second programming signal tofirst and second terminals of the CES device to place the CES device ina second impedance state; and applying a third programming signal tofirst and second terminals of the CES device to meet a second voltagecondition or a second current condition, or a combination thereof, forplacement of the CES device in the first impedance state, wherein apolarity of the first voltage condition is opposite a polarity of thesecond voltage condition, and a polarity of the first current conditionis opposite a polarity of the second current condition, and wherein adifference in a magnitude of the first voltage condition and a magnitudeof the second voltage condition or a difference in a magnitude of thefirst current condition and a magnitude of the second current condition,or any combination thereof, is responsive at least in part to anon-uniform application of a dopant to the one or more layers of the CEMdisposed between the first and second terminals.
 2. The method of claim1, wherein the first impedance state comprises a conductive or lowimpedance state and the second impedance state comprises an insulativeor low impedance state, and wherein the first programming signal isapplied in a first set operation, the second programming signal isapplied in a reset operation and the third programming signal is appliedin a second set operation.
 3. The method of claim 2, wherein the firstset operation is characterized by a first set voltage condition and thesecond set operation is characterized by a second set voltage condition,and wherein a magnitude of the first set voltage condition is greaterthan a magnitude of the second set voltage condition.
 4. The method ofclaim 2, wherein the first set operation is characterized by a first setcurrent condition and the second set operation is characterized by asecond set current condition, and wherein a magnitude of the first setcurrent condition is greater than a magnitude of the second set currentcondition.
 5. The method of claim 2, wherein application of the firstprogramming signal to the first and second terminals of the CES deviceand application of the third programming signal to the first and secondterminals of the CES device comprise injection of electrons in the CEMto achieve a sufficient concentration of electrons in the CEM to placethe CES device in the conductive or low impedance state.
 6. The methodof claim 1, wherein the first impedance state comprises an insulative orhigh impedance state and the second impedance state comprises aconductive or low impedance state, and wherein first programming signalis applied in a first reset operation, the second programming signal isapplied in a set operation and the third programming signal is appliedin a second reset operation.
 7. The method of claim 1, wherein the CESdevice is placed in the first impedance state or the second impedancestate based, at least in part, on a localization of electrons in theCEM.
 8. The method of claim 1, wherein the CES device comprises anon-volatile memory device.
 9. The method of claim 1, wherein a majoritya contiguous portion of the CEM is switchable between a conductive orlow impedance state and an insulative or high impedance state to placethe CES device in either the first impedance state or the secondimpedance state.
 10. The method of claim 9, wherein a majority acontiguous portion of the CEM is switchable between the conductive orlow impedance state and the insulative or high impedance stateresponsive to a Mott transition.
 11. The method of claim 9, wherein theCEM comprises a transition metal oxide that is non-uniformly doped withan extrinsic ligand.
 12. The method of claim 1, wherein the CES devicecomprises a variable impeder device, and wherein the first impedancestate imparts a first capacitance value and the second impedance stateimparts a second capacitance value.
 13. The method of claim 1, whereinthe one or more layers of the CEM comprise one or more contiguous layersof the CEM, and wherein the one or more contiguous layers of the CEMfurther comprises at least one layer of intrinsic CEM and one or moredoped layers of CEM.
 14. The method of claim 1, wherein the one or morelayers of the CEM comprise one or more contiguous layers of the CEM, andwherein the one or more contiguous layers of CEM comprise at least anintrinsic layer of CEM formed between an N-type doped layer of CEM and aP-type doped layer of CEM.
 15. The method of claim 1, wherein the one ormore layers of the CEM comprise one or more contiguous layers of theCEM, and wherein the one or more contiguous layers of the CEM compriseat least an intrinsic layer of CEM formed between a first P-type dopedlayer of CEM and a second P-type doped layer of CEM, wherein a thicknessof the first P-type doped layer of CEM is greater than a thickness ofthe second P-type doped layer of CEM.